Ignitor for fuel burner



Jan. 14, 1964 w. F. LANGE ETAL 3,117,619

IGNITOR FOR FUEL BURNER Filed Aug. 2, 1960 3 Sheets-Sheet l Inventors W/LL/HN AAA/6E 2 8 Attorney w. F. LANGE ETAL IGNITOR FOR FUEL BURNER Jam 14, 1964 Filed Aug. 2, 1960 3 Sheets- Sheet 2 Jan. 14, 1964 Filed Aug. 2,

' W. F. LANGE ETAL IGNI'I'OR FOR FUEL BURNER III... V'I'I'IA r" 5 Sheets-Shet s \Q Inventor WILL/AME (AA/6f $7'EPHEN/9: 5L OA/V B I I Attorney United States Patent lGNl l 6?. FUR FUEL BURNER W lliam F. Lange, Springdale, and Stephen A. Sloan, otamford Conn, as-signers to Peabody Engineering gorporatron, New York, NY, a corporation of New Filed Aug. 2, i960, Ser. No. 47,050

This invention relates to ignitors for fuel burners and more particularly to a gas-electric ignitor for multiple fuel burners adapted to burn liquid, gaseous or pulverized fuel.

An object of the invention is to provide an ignitor of the above type having novel and improved characteristics.

Another object is to provide an ignitor which is highly stable in operation and in which false shutdowns due to the ignition flame being blown away from the flame detector rod are eliminated.

Another object is to provide an ignitor of the above type so arranged and constructed that failure due to the presence of foreign particles either in the air stream or gas stream is substantially eliminated.

Another object is to provide an ignitor of the above type wherein the main ignition flarne utilizes air from the burner air register for effecting combustion.

Another object is to provide an ignitor of the above type having primary and main ignition means wherein the primary ignition means is doubly shielded from the air currents in the burner throat.

Another object is to provide an ignitor of the above type having primary and main flame retention nozzles and w erein the flame detector element is responsive only to the flame in the main retention nozzle.

Another object is to provide an ignitor of the above type in which the ignition elements can be removed without shutting down the burner.

Various other objects and advantages will be apparcut as the nature of the invention is more fully disclosed.

in accordance with the present invention, the ignitor comprises an outer barrel which projects through the wind box into 546 burner throat and is formed with a perforated end portion which constitutes a main flame retention nozzle. Within this barrel and recessed from the open end thereof is a primary flame retention nozzle having a tube having peripheral apertures and spaced from the outer barrel to provide an annular air passage therebetween. Gas for ignition is supplied through a distribution chamber disposed within the outer barrel and having a primary gas supply pipe terminating in an orifice. A main gas supply pipe passes along the annular passage and terminates in the main flame retention nozzle. Other gas supply pipes also connected with the distribution chamber supply gas ,0 diflferent portions of the annular passage above referred to. ignition means is provided to ignite the gas stream within the primary flame retention nozzle and a flame detector rod is disposed in the path of the flame in the main flame retention nozzle. The arrangement is such that ignition first takes place within the primary flame retention nozzle and the flame advances therefrom axially and outwardly into the annular space around the primary nozzle and into the zone of the main nozzle where it ignites the gas from the main gas supply pipe or pipes. The flame thus produced contacts the flame detector element within the main retention nozzle and combustion is completed as the flame advances beyond the ignitor into the burner throat.

The arrangement is such that when the main ignition flame is rich enough to ignite the burner it will be retained at least in part within the main flame retention nozzle in contact with the flame detector rod thereby preventing a flame failure indication being given which would be "ice the case if the ignitor flame were temporarily blown away from the flame detector rod. On the other hand if the flame is sufficiently lean to be blown away from the sensing rod it would be too lean to ignite the burner and a proper shutdown will occur. In addition, since th ignitor flame requires air from the burner for combustion any failure of the burner air would automatically affect the ignitor flame so as to result in cutting oil the fuel supply.

The nature of the invention will be better understood by referring to the following description taken in connection with the accompanying drawings wherein a specific embodiment has been set forth for purposes of illustration:

1G. 1 is a perspective view of an ignitor embodying the invention;

PEG. 2 is a plan section of a burner showing the ignitor in operating position;

FIG. 3 is an enlarged side elevation showing the fire room end of the ignitor with certain parts in section;

FIG. 4 is a similar side elevation with parts in vertical section showing the furnace end of the ignitor, FIGS. 3 and 4 together constituting a complete side elevation of the ignitor;

FIG. 5 is a vertical axial section taken on the line 55 of FIG. 2 but on a larger scale;

FIGS. 6 and 7 are transverse sections taken on the line 66 and 7-7 respectively of FIG. 5;

FIG. 8 is an end elevation of the ignitor taken on the same scale as FIG. 5;

FIG. 9 is a detail of a portion of the ignitor showing a different form of flame detector;

FIG. 10 is a transverse section taken on the line lt}ll of FIG. 9; and

FIG. 11 is an end elevation of FIG. 9.

Referring to the drawings more in detail, the invention is shown as applied to a burner for a furnace having a wall It? provided with a burner op-eriin ll lined by throat tile 12. A fire room wall 13 is spaced from the front wall 16 of the furnace to provide a wind box 14.

The burner embodies an air register 15 of standard construction having a set of peripheral air doors i6 and having a burner tube 17 extending across the wind box 14 and registering with the throat opening iii. A burner having a fuel barrel 1% is attached by a front plate 19 to the fire room wall 13 and extends axially through the air register 15 and air tube 17 and is provided with a tip 18a in the burner throat from which the cone of flame extends through the throat opening into the furnace chamber. The barrel 13 carries the usual diffuser plate 24 which may be adjustable axially. The usual gas ring 21 may be disposed around the burner throat for use with gaseous fuel. The apparatus thus far described is of standard form and only so much thereof has been set forth as is necessary to an understanding of the present invention.

The ignitor in accordance with the present invention comprises a fixed tube 25 extending between the fire room wall 13 and air tube 17. At its forward end, the tube 25 is secured to a casting 25 which in turn is carried by a Plate 27 attached to the fire room wall 13. At its rearward end the tube 25 is secured as by welding to the air tube 17.

The ignitor includes an outer barrel 28 which is positioned within the tube 25 and projects into the air tube 17 and into the throat 11 at its rearward end. The projecting end of the barrel 28 is provided with perforations 29 to form a main flame retention nozzle 29.1.

A housing 30 is disposed within the barrel 2% at a point intermediate of the length of the barrel. This housing 39* is formed with a pair of upper segments 31 the ignitor shown in and 32 and a lower segment 33, the segments having arcuate peripheral surfaces adapted to slide within the barrel 28 and being spaced apart peripherally to provide air passages therebetween. A gas admission pipe 34 extends forwardly from the housing 3% and is adapted to supply gas for ignition purposes to the hollow interior of the housing which serves as a distribution chamber. A mixing tube 3? is carried by the rear wall 39 of the housing 3d and extends axially along the barrel 2% and concentric therewith and is spaced therefrom to form an annular passage 4-!) therebetween. At its forward end, adjacent the wall 39, the tube 38 is cut away as at 41 to provide an opening for the entrance of air. At its rearward end the tube 3% is provided with a fitting 42 having a frusto-conical wall formed with a ring of holes 44 and an open end 45. The fitting 42 usually is also provided with a rearwardly projecting extension 6 having at its end an inwardly turned lip 47 to assist in providing the proper turbulence. Extension '46 and lip 47 may be omitted if desired. A tube at; having its rearward end provided with perforations 49 is disposed over the fitting 42. and secured thereto by welding and projects axially along the barrel 28 to form a primary flame retention nozzle 4% within the main retention nozzle 2%. A primary gas supply pipe 5%} is carried by the rear wall 39 or" the housing 3% and projects axially into the tube 38 beyond the cut away portion 41 and is provided at its rearward end with a gas tip 51.

A pipe 54- extends rearwardly from the lower portion of the wall 39 in the space between the barrel 28 and the tube 38 with its rearward end bent inwardly and extending through a slot 55 in the wall of the tube 48 with its rearward end projecting axially and centrally beyond the rearward end or" the primary nozzle 4%. The pipe 54 supplies gas for ignition from the distribution chamber in the housing 3% to the main flame retention nozzle 29a. A plurality of pipes 5e are connected to the rear wall 39 of the housing 36 and extend in the space 40 between the barrel 28 and the tube 38 to the zone of the primary nozzle 49a. These pipes 56 supply additional gas to the annulus 49 for stabilizing the combustion. Two such pipes have been shown. However, the number of pipes may be selected in accordance with the requirements of a particular installation. The main gas supply pipe 54 has been shown as extending beyond the pipes 56 into the main nozzle 2%. However, the pipes 54 and 56 may all terminate in the annular passage it if desired and their ends may be swaged to form a restricted tip opening for the emission of gas, the opening being dependent upon the gas pressure and the desired quantity of flow.

An ignition electrode 66 extends along the barrel 23 and through the segment 31 of the housing 3% with its rearward end 61 bent inwardly and extending into a suitable opening 62 in the nozzle 49a. The electrode is suitably insulated and an electrical potential is supplied thereto from a source not shown to cause an ignition spark to extend from the rearward end 61 of the electrode to the grounded nozzle 49a.

A flame detection rod 65 extends through the barrel 28 and through the segment 32 of the housing 3!} and through the annular space ll) between the barrel 28 and the pipe 3 8 with its rearward end inclined downwardly to cross the axis of the barrel 2?; beyond the open end of the primary nozzle 49a with its rearward end projecting slightly beyond the end of the main nozzle 29a. A perforated disc 66 is disposed within the barrel 28 beyond the housing 3% to support the various pipes above mentioned and act as a radiation shield.

At its forward end the barrel 28 extends through a packing gland 68 carried by the casting 25 and is provided with an end flange 69 which is attached to a flange 79 carried by an annular manifold 71 having a rearwardly inclined wall '72 formed with apertures 73 for the supply of air into the barrel. High pressure air may be supplied to the manifold by a pipe 74. Air at lower pressure i may also be supplied to the interior of the barrel 28 by a pi e 75.

Disposed over the end of the manifold 71 is a housing 76 which if bolted to the manifold 71 by bolts 76:: and forms an end closure for the barrel 2 8. The housing 76 carries a box 77 in which the various control valves and relays are mounted and which carries a control panel 73 provided with a plurality of signal lights '78, 8% and 81 and with a three position switch 82 by which the operation of the ignitor is controlled. A remote control panel may be used if desired. The housing 76 carries the gas admission pipe 3d the gas supply to which is controlled by a solenoid valve 85.

In operation for ignition purposes gas for ignition is supplied through the pipe 34 to the distribution chamber within the housing 3% thence through the pipe 5% to the primary flame retention nozzle 49a. The gas issuing from the tip 51 within the pipe 33 serves as an inspirator to draw air from the burner throat inwardly through at least some of the perforations Z9, thence forwardly through the annular space ll} and through the cut away portion 41 of the pip-e 33 where it mixes with the gas discharged from the tip 51 of the pipe 54? to form a combustible mixture which is ignited by a spark from the end 61 of the electrode 66. The primary gas-air mixture passes axially out of the open end 45 of the fitting 4-2 and also outwardly through the perforations 44 where it is deflected by the annular wall 46 and the lip 47 to combine with the mix ture issuing from the open end 45. This combined mixture, after ignition, forms a flame which then passes into the zone within the primary retention nozzle 49:: and is projected axially from the end of the nozzle 4% into the zone within the main retention nozzle 2% where it ignites the gas from the main supply pipe 54 and outwardly through the perforations 49 into the annular passage 49 where it ignites the gases issuing firom the pipes 56 for reinforcing the flame within the main nozzle 29a. The flame thus produced constitutes the main ignition flame and contacts the flame detection rod 65. This flame passes outwardly from the end of the main nozzle 29a and also outwardly through the rearward perforations 29 into the burner throat for igniting the fuel burner.

It will be noted that the main ignition flame is shielded within the main flame retention nozzle 29a and the primary flame is doubly shielded by the main flame retention nozzle 2% and by the primary flame retention nozzle 4% so that the ignition flame cannot be blown out by the strong air current within the burner throat.

The primary flame retention nozzle 4% constitutes a two stage member. In it the gas-air mixture within the cone shaped fitting 42 is discharged not only through open end 45 but also through orifices 44 in the form of jets. These jets ordinarily are shielded and slowed down by the annular wall 46 and lip 47 so as to produce suitable turbulence. However, if so desired wall 46 and lip 47 may be omitted from the structure. When these various streams are ignited by electrode 61 the series of burning streams from the orifices 44 serve to stabilize and retain the flame leaving at 45. The whole primary flame body then enters the zone within the primary retention nozzle 4% which serves as a shield and as a mixing and retention nozzle. It is so arranged that the primary ignition flame thus produced cannot be blown out by the air from the main air register. Thus, there is always present flame for igniting and maintaining ignition of the gas issuing from the pipes 54 and 56 in the zone within the main flame retention nozzle 2%.

The elements within the barrel 28 can be removed, without removing the barrel 28 itself which may be relatively lon and heavy, by removing the bolts 76a which secure the housing 76 to the manifold 71. The housing 76 together with the gas admission pipe 34-, the housing 3t} and the parts connected thereto may then be withdrawn from the barrel 28. Before the internal members have been thus removed air under pressure should be supplied to the manifold 71 by the pipe 7 to discharge air through the apertures 73 into the barrel 2% and thereby prevent the discharge of furnace gases through the pipe 28 into the fire room when the furnace is operating at above atmospheric pressure.

in the above construction it will be noted that gas which is supplied through the gas admission pipe 34 to the distribution chamber in the housing undergoes two 90 changes in direction before it enters the primary gas supply pipe 50. This prevents any sediment or extraneous particles from being drawn into the small primary gas supply pipe and clogging the nozzle. Such particles, however, can readily pass through the main gas supply pipe 54 without clogging the same. Adso, any air which is drawn in through the perforations 29 of the outer barrel 28 is required to make a 180 turn before it enters the opening 41 and mixes with the gas in the primary retention nozzle 49a to form a combustible mixture. Hence, any foreign particles tend to be thrown out of the air stream without entering the primary retention nozzle.

In a regenerative type air heater it would be very easy for foreign material in the form of ash from the burning of the fuel to get into the combustion air stream and thus serve as a source of contamination for the ignitor. Since in the present case only a small portion of the air for supporting combustion of the ignitor flame enters the ignitor from the burner throat the tendency of the surfaces of the ignitor and in particular of the ignition electrode and flame detector rod to become coated with foreigri material from the air stream in the main burner is materially reduced. This is due to the fact that air within the ignitor barrel is only required to support the combustion of the small primary ignition flame and to initiate the combustion of the main ignition lame. Full cornbustion of the main ignition flame takes place only after the main ignition flame has left the nozzle 2% and is within the air stream in the burner throat. When pulverized coal is being burned, it is desirable to introduce low pressure air through the pipe 75 to pass through the barrel 28 into the burner throat so as to reduce the amount of pulverized coal which might otherwise be drawn into the ignitor. Such air supply may be m- "ntained constantly during the burning period and utter the ignitor has been shut off so as to purge the ignitor barrel. The air supply from this pipe 7'5 in such case could serve all or a portion of the combustion air for the primary ignitor. Suitable axial adjustment of the barrel may be made to position the ignition frame as required in the burner throat.

The flame detector rod is of course connected to suitable relays or the like to close t e main gas supply when flame failure is indicated. Such connections being well known in the art are not shown herein. "the three position switch 82 is normally set to oil" position. in a second position the electrical connections are made an the system is ready for operation. in the fllird position, the gas supply is open and fir'mg takes place. Signal lights 79, 8t and 81 may be connected to indicate the three positions of the switch 82. In certain instances, it may be desirable to retract the tip of the barrel 2% after ignition has taken place in the burner throat although it is usual to so position the ignitor tip that it may remain in place at all times and is ready for use as an ignitor when required. FIGS. 9, l0 and 11 show a modified form of flame detector rod wherein a pair of insulated rods 88 are carried 'by segments 31 and 32 of the housing and project through the passage 4% with their ends attached to a ring 89 disposed in the passage adjacent the end of the primary nozzle 4%. A plurality of flame detector rods 94 are carried by the ring 89 and extend into the main nozzle 2% with their ends curved inwardly and joined to form a cage-like structure. This structure may replace the flame detector rod previously described and has the advantage of being somewhat more rigid particularly when subjected to high temperatures.

A specific embodiment of the ignitor has been shown for purposes of illustration. It is to be understood 10wever that various changes and modifications may be therein to adapt the ignitor to specific uses as will be apparent to a person skilled in the art.

What is claimed is:

1. An ignitor for a fuel bur er comprising an ignitor barrel terminating in a main retention nozzle, a primary flame retention nozzle disposed within said barrel in advance of said main nozzle, a housing within said barrel carrying said primary nozzle, a plurality of rods mounted on said housing and extending into the zone around said primary nozzle, an annular member disposed around said primary nozzle carried by said rods and a plurality of flame detector elements mounted on said member and extending into the zone within said main nozzle, said elements having end joined at the axis of said barrel in the path of a flame within said rnain nozzle.

2. An ignitor for a fuel burner including an air tube having a turbulent air flow therein, compris 1 a main barrel closed at its forward end and terminating rearwardly in a perforate cylindrical element forming a main flame retention nozzle to be exposed to said turbulent air flow, a primary gas supply pipe having a discharge tip disposed concentric with said barrel, a mixing tube concentric with said primary gas supply pipe spaced radially from said gas supply pipe and inwardly from said barrel to provide separate inner and outer annular air passages and having means to close the forward end of said inner =air passage, said mixing tube having an opening in advance of said tip for admitting air from said outer annular passage to said inner annular passage for mining with the gas from said tip to form a combustible mixture, whereby said tip serves as an inspirator to induce air flow from said main nozzle forwardly through said outer annular passage and into said inner annular passage and a perforate cylindrical member carried said mixing tube and extending rearwardly or said tip to form a primary flame retention nozzle which is spaced axially in advance of the end of said main nozzle to leave a gas ignition zone within said main nozzle, a main gas supply pipe within said barrel and outside of said m'ming tube and said perforate cylindrical member having a tip disposed to supply raw gas to the zone of said main nozzle for combustion, an ignition electrode terminating within said primary nozzle and adapted to ignite the gas widtin said primary nozzle, and a lame sensing element disposed in said main nozzle in the path of the flame therein whereby the primary ignitor flame is first formed within said primary retention nozzle and a main ignitor flame is then formed within said retention nozz e and is sensed by said flame-sensing element.

3. An ignitor as set forth in claim 2 in which said main gas supply pipe extends axially through said outer annular passage and has a tip disposed coaxially with said primary nozzle.

4. An ignitor as set forth in claim 2' in which at least one main gas supply pipe is disposed in said outer annu ar passage and is formed with a tip discharging gas into said passage.

5. An ignitor as set forth in claim 2 in which a hollow housing forming a gas distribution chamber is disposed within said barrel in advance of said nozzles, a gas admission pipe is connected to supply gas to said chamber and said primary gas supply pipe and said main gas supply pipe are connected to receive gas from said chamber.

6. An ignitor as set forth in claim 5 in which said primary nozzle, said sensing element and said ignition electrode are mounted on said housing and said housing is slidably mounted for retraction within said barrel.

7. An ignitor as set forth in claim 5 in which said admission pipe is connected to supply gas near the riphery of said distribution chamber, said primary ga supply pipe is connected to receive gas from a point at the axis of said chamber, and said main gas supply pipe "Cl Ga 7 is disposed in substantial alignment with said gas admis sion pi e whereby particles in the gas stream tend to pass through said main pipe and are inhibited from flowing through said primary pipe.

8. An ignitor for a fuel burner including an air tube having a turbulent air flow therein, comprising a barrel terminating rearwar .ly in a perforate cylindrical element open at its rearward end and forming a main flame retention nozzle to be exposed to said turbulent air flow, a primary flame retention nozzle disposed within said barrel axially forward of said main flame retention nozzle and comprising a second perforate cylindrical element open at its rem'ward end for the passage of a primary ignition flame, means including a mixing tube terminating in said primary flame retention nozzle adapted to supply thereto a combustible mixture of air and fuel, a main gas supply pipe within said barrel disposed to supply raw gas to the zone of said main nozzle, said barrel being imperforate forward of said main nozzle and closed at its forward end whereby the air for combustion in said main nozzle is derived solely from the air Within said air tube, an ignition electrode terminating within said primary nozzle, and a flame sensing element disposed in said main nozzle in the path of the flame therein whereby the primary ignitor flame is first formed within said primary retention nozzle and a main ignitor flame is then formed within said main retention nozzle and is sensed by said flame-sensing element.

9. in combination with a fuel burner including a front plate and an air tube having a turbulent air flow therein, an ignitor comprising a barrel extending through the wall of said air tube with its forward end exposed in advance of said :front plate and terminating rearwardly in a perforate cylindrical element forming a main flame retention nozzle to be exposed to said turbulent air flow, a primary gas supply pipe having a discharge tip disposed concentric with said barrel, a mixing tube concentric with aid primary gas supply pipe spaced radially from said barrel to provide separate inner and outer annular air passages and having means to close the forward end of said inner air passage, said mixing tube having an opening in advance of said tip for admitting air from said outer annular passage to said inner annular passage for mixing with the gas from said tip to form a combustible mixture, whereby said tip serves as an inspirator to induce flow from said main nozzle forwardly through said outer annular passage and into said inner annular passage and a perforate cylindrical member carried by said mixing tube and extending rearwardly of said tip to form a primary flame retention nozzle which is spaced axially in advance of the end of said main nozzle to leave a gas ignition zone Within said main nozzle, a main gas supply pipe within said barrel and outside of said mixing tube and said penforate cylindrical member having a tip disposed to supply raw gas to the zone of said main nozzle for combustion, an ignition electrode termihating within said primary nozzle and adapted to ignite the gas Within said primary nozzle, and a flame sensing element disposed in said main nozzle in the path of the flame therein whereby the primary ignitor flame is first formed within said primary retention nozzle and a main ignitor flame is then formed within said main retention nozzle and is sensed by said flame-sensing element, a hollow housing forming a gas distribution chamber dis posed Within said barrel in advanw of said nozzle, 21 gas admission pipe connected to supply gas to said chamber, said primary gas supply pipe and said main gas supply pipe being connected to receive gas from said chamber, a second housing closing the forward end of said ban'el whereby said gas admission pipe and the elements connected therewith may be retracted axially with said second housing and an annular manifold connected to said barrel in advance of said front plate and adapted to supply air into said barrel for preventing the escape of furnace gases when said second housing is removed from said barrel.

References Cited in the file of this patent UNITED STATES PATENTS 

2. AN IGNITOR FOR A FUEL BURNER INCLUDING AN AIR TUBE HAVING A TURBULENT AIR FLOW THEREIN, COMPRISING A MAIN BARREL CLOSED AT ITS FORWARD END AND TERMINATING REARWARDLY IN A PERFORATE CYLINDRICAL ELEMENT FORMING A MAIN FLAME RETENTION NOZZLE TO BE EXPOSED TO SAID TURBULENT AIR FLOW, A PRIMARY GAS SUPPLY PIPE HAVING A DISCHARGE TIP DISPOSED CONCENTRIC WITH SAID BARREL, A MIXING TUBE CONCENTRIC WITH SAID PRIMARY GAS SUPPLY PIPE SPACED RADIALLY FROM SAID GAS SUPPLY PIPE AND INWARDLY FROM SAID BARREL TO PROVIDE SEPARATE INNER AND OUTER ANNULAR AIR PASSAGES AND HAVING MEANS TO CLOSE THE FORWARD END OF SAID INNER AIR PASSAGE, SAID MIXING TUBE HAVING AN OPENING IN ADVANCE OF SAID TIP FOR ADMITTING AIR FROM SAID OUTER ANNULAR PASSAGE TO SAID INNER ANNULAR PASSAGE FOR MIXING WITH THE GAS FROM SAID TIP TO FORM A COMBUSTIBLE MIXTURE, WHEREBY SAID TIP SERVES AS AN INSPIRATOR TO INDUCE AIR FLOW FROM SAID MAIN NOZZLE FORWARDLY THROUGH SAID OUTER ANNULAR PASSAGE AND INTO SAID INNER ANNULAR PASSAGE AND A PERFORATE CYLINDRICAL MEMBER CARRIED BY SAID MIXING TUBE AND EXTENDING REARWARDLY OF SAID TIP TO FORM A PRIMARY FLAME RETENTION NOZZLE WHICH IS SPACED AXIALLY IN ADVANCE OF THE END OF SAID MAIN NOZZLE TO LEAVE A GAS IGNITION ZONE WITHIN SAID MAIN NOZZLE, A MAIN GAS SUPPLY PIPE WITHIN SAID BARREL AND OUTSIDE OF SAID MIXING TUBE AND SAID PERFORATE CYLINDRICAL MEMBER HAVING A TIP DISPOSED TO SUPPLY RAW GAS TO THE ZONE OF SAID MAIN NOZZLE FOR COMBUSTION, AN IGNITION ELECTRODE TERMINATING WITHIN SAID PRIMARY NOZZLE AND ADAPTED TO IGNITE THE GAS WITHIN SAID PRIMARY NOZZLE, AND A FLAME SENSING ELEMENT DISPOSED IN SAID MAIN NOZZLE IN THE PATH OF THE FLAME THEREIN WHEREBY THE PRIMARY IGNITOR FLAME IS FIRST FORMED WITHIN SAID PRIMARY RETENTION NOZZLE AND A MAIN IGNITOR FLAME IS THEN FORMED WITHIN SAID MAIN RETENTION NOZZLE AND IS SENSED BY SAID FLAME-SENSING ELEMENT. 